In wireless communication, OFDM (Orthogonal Frequency Division Multiplexing) has been viewed as a popular technical solution with high spectrum efficiency for realizing high data rate, especially in 4G (4th Generation) wireless communication systems, because of its effective use of frequency band and simple implementation. Typically in an OFDM system, available bandwidth is divided into a number of discrete channels, which are overlapping and orthogonal to each other. Each discrete channel is defined as a subcarrier and has a well-defined frequency. Each subcarrier carries modulated symbols, the amplitude and/or phase of which represent encoded information. At receiver end, received OFDM symbols can be demodulated only after the starting time of each OFDM symbol is identified. Therefore, time synchronization is required to identify the timing of the symbol. However, synchronization error exists due to channel fading, interference, noise and so on.
There are two kinds of synchronization error, early synchronization error and late synchronization error. The impacts of the early synchronization error to subsequent blocks can be ignored as long as the estimated starting time lies in the range of cyclic prefix of the symbol. The late synchronization error is more harmful than the early synchronization error, as there is no effective protection provided against this kind of synchronization error.
In practice, the estimated symbol starting time is typically shifted backwards a certain amount within the cyclic prefix to reduce the probability of the late synchronization error. However, for an OFDM system with multi-path fading channel, the late synchronization error is more likely to occur. This is due to the fact that the starting time of the signal transmitted through the strongest path (rather than the starting time of the signal transmitted through the weaker path with the smallest propagation delay) is typically taken as the starting time of the received symbol for time synchronization. Thus, the above-mentioned method of shifting the estimated symbol starting time can provide little help in this situation, because the strongest path may have much larger propagation delay than the first path.
In order to avoid the above-mentioned problem, prior solutions focus on special designed training sequences in the preamble field or longer cyclic prefix. However, such solutions are disadvantageous in that modification of the format of preamble field or data field is needed and the modification may have conflict with specifications in communication standards.
Another prior solution focuses on joint synchronization and channel estimation. Joint estimation can obtain more accurate result than separate estimation, but the computation complexity of joint estimation is much more than that of separate estimation. More complexity results in higher cost. An improved joint estimation is proposed to improve channel estimation by weighted averaging multiple training sequences based on estimated power of interference. But, in the system with the late synchronization error, all the received training sequences involve great interference, and therefore, the method of weighted averaging will not bring improvement.
MIMO (Multiple Input Multiple Output) OFDM system is more sensitive to synchronization error than SISO (Single Input Single Output) OFDM system, because synchronization error in one receiver can not only introduce interference to it own but also confuse other receivers. Because there are different timing offsets between multiple transmitters and a single receiver, ideal synchronization or early synchronization for signal from one transmitter often becomes late synchronization for signal from another transmitter. So synchronization error is difficult to avoid in MIMO OFDM system even if the clocks of some transmitter and receiver are perfectly synchronized.